Lithographic printing typically involves the use of a so-called printing master such as a printing plate which is mounted on a cylinder of a rotary printing press. The master carries a lithographic image on its surface and a print is obtained by applying ink to said image and then transferring the ink from the master onto a receiver material, which is typically paper. In conventional lithographic printing, ink as well as an aqueous fountain solution (also called dampening liquid) are supplied to the lithographic image which consists of oleophilic (or hydrophobic, i.e. ink-accepting, water-repelling) areas as well as hydrophilic (or oleophobic, i.e. water-accepting, ink-repelling) areas. In so-called driographic printing, the lithographic image consists of ink-accepting and ink-abhesive (ink-repelling) areas and during driographic printing, only ink is supplied to the master.
Printing masters are generally obtained by the image-wise exposure and processing of an imaging material called plate precursor. A typical positive-working plate precursor comprises a hydrophilic support and an oleophilic coating which is not readily soluble in an aqueous alkaline developer in the non-exposed state and becomes soluble in the developer after exposure to radiation. In addition to the well known photosensitive imaging materials which are suitable for UV contact exposure through a film mask (the so-called pre-sensitized plates), also heat-sensitive printing plate precursors have become very popular. Such thermal materials offer the advantage of daylight stability and are especially used in the so-called computer-to-plate method (CtP) wherein the plate precursor is directly exposed, i.e. without the use of a film mask. The material is exposed to heat or to infrared light and the generated heat triggers a (physico-)chemical process, such as ablation, polymerization, insolubilization by cross-linking of a polymer or by particle coagulation of a thermoplastic polymer latex, and solubilization by the destruction of intermolecular interactions or by increasing the penetrability of a development barrier layer.
Although some of these thermal processes enable platemaking without wet processing, the most popular thermal plates form an image by a heat-induced solubility difference in an alkaline is developer between exposed and non-exposed areas of the coating. The coating typically comprises an oleophilic binder, e.g. a phenolic resin, of which the rate of dissolution in the developer is either reduced (negative working) or increased (positive working) by the image-wise exposure. During processing, the solubility differential leads to the removal of the non-image (non-printing) areas of the coating, thereby revealing the hydrophilic support, while the image (printing) areas of the coating remain on the support.
Typically, for a positive-working thermal plate, a dissolution inhibitor is added to a phenolic resin as binder whereby the rate of dissolution of the coating is reduced. Upon heating, this reduced rate of dissolution of the coating is increased on the exposed areas compared with the non-exposed areas, resulting in a sufficient difference in solubility of the coating after image-wise recording by heat or IR-radiation. Many different dissolution inhibitors are known and disclosed in the literature, such as organic compounds having an aromatic group and a hydrogen bonding site or polymers or surfactants comprising siloxane or fluoroalkyl units.
The known heat-sensitive printing plate precursors typically comprise a hydrophilic support and a coating which is alkali-soluble in exposed areas (positive working material) or in non-exposed areas (negative working material) and an IR-absorbing compound. Such coating typically comprises an oleophilic polymer which may be a phenolic resin such as novolac, resol or a polyvinylphenolic resin. However, these plates suffer on a lack for resistance against press chemicals and the printing run length of these plates needs to be improved.
Therefore, in order to improve the printing run length, the phenolic resin is chemically modified whereby the phenolic monomeric unit is substituted by a group such as described in WO99/01795, EP 934 822, EP 1 072 432, U.S. Pat. No. 3,929,488, EP 2 102 443, EP 2 102 444, EP 2 102 445, EP 2 102 446. The phenolic resin can also been mixed with other polymers such as an acidic polyvinyl acetal as described in WO2004/020484 or a copolymer comprising sulfonamide groups as described in U.S. Pat. No. 6,143,464 or other polymeric binders as described in WO2001/09682, EP 933 682, WO99/63407, WO2002/53626, EP 1 433 594 and EP 1 439 058. However, as a result of these modifications of the phenolic resin or the addition of other binders to the phenolic resin, the quality of printing plates is usually reduced, e.g. a reduced sensitivity of the plate on image-wise exposing or a reduced developing latitude. This means that the difference between the rate of dissolution of the exposed areas and the non-exposed areas is reduced. This may result in an insufficiently removal of the coating at the exposed areas, i.e. an insufficient clean-out of the plate, and, as a result, toning may occur on the press. In another possibility, this reduced difference may also result in a reduced coating thickness of the coating at the non-exposed areas resulting in a reduced printing performance such as a reduced ink acceptance of the printing areas or a reduced printing run length.
Also, positive-working printing plates are described in the prior art which comprise other polymeric binders, usually alkali soluble resins, in an intermediate layer between the heat-sensitive recording layer and the support. In these plates, the heat-sensitive coating together with the intermediate layer are removed at the exposed areas and printing plates can be obtained having an improved clean-out and an improved chemical resistance against press chemicals and printing run length. Typical examples of positive-working thermal plate materials having such a two layer structure are described in e.g. EP 864420, EP 909657, EP-A 1011970, EP-A 1263590, EP-A 1268660, EP-A 1072432, EP-A 1120246, EP-A 1303399, EP-A 1311394, EP-A 1211065, EP-A 1368413, EP-A 1241003, EP-A 1299238, EP-A 1262318, EP-A 1275498, EP-A 1291172, WO2003/74287, WO2004/33206, EP-A 1433594 and EP-A 1439058. However, these plates of the prior art suffer on undercutting, i.e. partially dissolving of the intermediate layer at the non-exposed areas, especially at the edges of the printing areas due to the poor resistance of the intermediate layer for the alkaline developer. As a result of this undercutting, it is difficult to form highly sharp and clear images, particularly highlights, i.e. fine images comprising a dot pattern or fine lines are difficult to be reproduced.
In a high quality plate it is advantageous that such highlights can be reproduced within a sufficient developing latitude, i.e. small fluctuations in developing time and temperature and in developer conductivity do not substantially affect the image formed on the plate and this developing latitude is obtained when the difference in dissolution rate is improved. Therefore, the inventors of the present invention found a new compound which is capable of improving the lithographic differentiation between the exposed and non-exposed printing areas and an improved clean-out at the exposed areas and a high alkaline resistance at the non-exposed areas.
WO 2002/53626 and WO 2002/53627 disclose an imageable element comprising a thermally sensitive supramolecular polymer which exhibits an increased solubility in an aqueous developer solution upon heating.